Goo Gone citrus in the sprayer pump bottle (orange fluid; looks like a windex pump bottle). Spray it on, let it soak for 5-10 minutes. Peel the decal off slowly in a straight line, rub the rest of the glue off.
We had "subdued" cars with giant reflective stripes on the back ... really not the point. With permission they were removed; the new ones did not have them at all.
These decals were fairly new, like months old, but came off easy with this method. Did it in the do-it-yourself car wash bay and actually blew the rest of the coagulated glue off with soap and water spray.
I did five cars plus a bit and still have over half a bottle left. Smells good, too.